This is correct. Autosaves are stored every few minutes (dependent on what’s set in Preferences). Each time Dorico autosaves, the previous autosaved version is trashed.
When you close a project correctly (telling Dorico to either save or discard changes to the file), Dorico purges the autosave folder entirely, dumping its contents into the Recycle Bin.
Oh that’s a good idea of theirs (yet another one!) I just verified it’s true on Mac’s too, looks like they recycle everything including all the intermediate saves and backup.